1. Meiosis and
Sexual Reproduction

2. SEXUAL REPRODUCTION
Sex cells formed by meiosis
Requires 2 parents
Offspring not identical
Meiosis
The process that results in the formation of gametes
Gametes
Sex cells, egg or sperm, which have haploid chromosomes
Haploid
Haploid means HALF, as in the number of chromosomes,
written as n

3. The union of an egg and a sperm
Fertilization n 2n
The union of an egg and a sperm
Zygote
The cell that is formed when the egg becomes fertilized by the sperm. This results in a full set of chromosomes.
Somatic Cells
All body cells except for sex cells that have diploid chromosomes
Diploid
Diploid means they have chromosome found in pairs,
written as 2n

4. Homologous Chromosomes
Similar chromosomes that are found in pairs. The paired chromosomes come from the mother and father.
* Human body cells have 46 chromosomes each
* Human body cells have 23 homologous pairs
Meiosis and Fertilization
Important for survival of many species, because these processes result in genetic variation of offspring.
Meiosis
A kind of cell division that results in gametes (sex cells) with half the number of chromosomes.

5. A. Gametes do not have homologous pairs, but instead they have only half the chromosomes from each pair. Gametes are referred to as haploid (n).
B. When fertilization occurs, the zygote will have a normal number of chromosomes (2n).
C. In meiosis, specialized cells divide 2 times producing daughter cells.

6. Phases of Meiosis
(2 Cell divisions – Meiosis I & Meiosis II)
MEIOSIS I
Prophase I
Homologous chromosomes pair together to form tetrads
During Prophase I, crossing over occurs between homologous chromosomes and genes are exchanged causing genetic variation

7. Metaphase I
Spindle fibers attach to chromosomes and line them up in the middle of the cell.
Anaphase I
The fibers pull the homologous chromosomes apart. This is called disjunction. Chromosomes move toward opposite sides of the cell.

8. Telophase I
During Telophase I and cytokinesis, the nuclear membrane forms and the cell splits into two new cells.
**** The Chromosome number is reduced to half during Meiosis I forming haploid (n) daughter cells

9. MEIOSIS II
Prophase II
Spindle fibers form and attach to double stranded chromosomes
Metaphase II
Chromosomes line up in the middle of the cell

10. Anaphase II
Sister chromatids separate and move towards opposite sides of the cell
Telophase II
Cells divide, resulting in 4 haploid cells – gametes (sex cells)

11. Males form 4 haploid cells which develop into 4 spermK
Females form 4 haploid cells which develop into 1 egg and 3 small polar bodies
Egg
Polar Bodies
Small polar bodies are not used and break down.

12. Meiosis Parent cell – chromosome pair Chromosomes copied
1st division - pairs split
2nd division – produces 4 gamete cells with ½ the original no. of chromosomes

13. Meiosis – mouse testes
Parent cell
1st division
2nd division
4 gametes

14. Figure 13.7 The stages of meiotic cell division: Meiosis I

15. Figure 13.7 The stages of meiotic cell division: Meiosis II

16. One Way Meiosis Makes Lots of Different Sex Cells (Gametes) – Independent Assortment
Independent assortment produces 2n distinct gametes, where n = the number of unique chromosomes.
In humans, n = 23 and 223 = 6,000,0000.
That’s a lot of diversity by this mechanism alone.

18. Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over
Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.

19. Mitosis vs. Meiosis

20. The Key Difference Between Mitosis and Meiosis is the Way Chromosomes Uniquely Pair and Align in Meiosis
Mitosis
The first (and distinguishing) division of meiosis

21. Mitosis vs. Meiosis

22. Boy or Girl? The Y Chromosome “Decides”
X chromosome
Y chromosome

23. Boy or Girl? The Y Chromosome “Decides”

24. Meiosis – division error
Chromosome pair

25. Meiosis error - fertilization
Should the gamete with the chromosome pair be fertilized then the offspring will not be ‘normal’.
In humans this often occurs with the 21st pair – producing a child with Downs Syndrome

26. 21 trisomy – Downs Syndrome
Can you see the extra 21st chromosome?
Is this person male or female?